I have a pretty standard method for painting tarmac. It basically involves splodging Precision Paints tarmac and faded tarmac paint around with a sponge, then finishing up with some talcum powder applied with a sponge and finished with a big brush.
This method has always worked well and given excellent results, so that is how I planned to handle the Selly Oak road.
Of I splodged, but the effect wasn't working.At least on the main road.
On the side road to the woodyard, it was as good as ever. This was weird.
My only thought is that the side road is plaster, which is porus. The main road is MDF, which isn't. Thinking about this a bit more, I normally work on a card base, another porous material.
Basically, the colours weren't blending properly. The result wasn't terrible, just not as good as normal.
With a hint of desperation, I gave the road a very light coat of grey primer to tie everything together. The can was used well above the surface and very lightly. The result is a lot better. You can see the colour variations, but they are subtle, as they should be.
It just goes to prove that we all learn all the time. Since doing this I've looked at a bit more information and see that many people are hitting the weathering powders for this job - something I've done myself in the past. I'll see how the road looks as the project progresses, but there is a pack of grey pastels on the shelf...